Galvanization, or galvanizing, is a common process that has long been used to protect steel parts by applying a protective coating of zinc to the surface of the steel. During the galvanizing process, steel parts are first degreased to remove dirt, oil and organic substances. After degreasing, the parts are rinsed with water and then pickled in a dilute solution of hydrochloric or sulfuric acid to remove iron oxides and mill scale. After rinsing again, the steel parts are dipped in a flux tank containing a solution of zinc chloride and ammonium chloride. The flux solution is slightly acidic, so it removes any remaining oxidation and creates a protective coating on the steel before it is dipped into a kettle of molten zinc where a final protective layer of zinc forms on the parts.
After multiple uses, the solution in the flux tank becomes unsuitable for further use due to contamination with metals from the steel parts, as well as chemicals that may be carried over from the pickling solution. The most common contaminants are ferrous iron (Fe+2 ions) and sulfates (in galvanizing operations that use sulfuric acid for pickling). Disposal of the contaminated flux as hazardous waste is usually prohibitively expensive. The flux may be neutralized to make it less hazardous, but this creates large volumes of sludge for disposal. Both of these approaches waste large amounts of zinc ammonium chloride salts which are dissolved in the flux solution and are necessary for proper fluxing of parts during the galvanizing process.
Rather than disposing of the contaminated flux solution, another approach is to remove the contaminants so the flux can be reused. A fundamental challenge with this approach is removing dissolved contaminants such as ferrous and sulfate ions while retaining the valuable zinc ammonium chloride which is also dissolved in the flux.
FIG. 1 illustrates a prior art system for removing ferrous iron from a galvanizing flux bath of zinc ammonium chloride. Contaminated solution from the bottom of the flux bath 10 is pumped to a first continuously stirred treatment tank 12 where hydrogen peroxide reacts with the soluble ferrous iron (Fe+2) to form insoluble ferric iron (Fe+3). Ammonium hydroxide is also added to maintain the pH at a level that maximizes precipitation of the iron while minimizing precipitation of the zinc. The treated solution is then transferred to a second continuously stirred treatment tank 14 where a polymer is added to facilitate precipitation of the insoluble ferric iron. The solution is allowed to overflow into a surge tank 16 and is then pumped into a clarifier 18 which is equipped with a tube settler. Supernatant flows back to the flux bath 10 while sludge is compacted in a filter press 20. Filtrate from the filter press flows back to the first treatment tank while compacted solids are sent to a disposal facility.
Although the system of FIG. 1 has been used for decades, it has multiple drawbacks that have become increasingly problematic in recent years. First, hydrogen peroxide is a hazardous chemical to work with. In addition to being highly toxic and corrosive, it can also be explosive. Handling hydrogen peroxide safely requires proper training and safety equipment which add cost, time and complexity to a galvanizing operation. Moreover, even relatively concentrated solutions of hydrogen peroxide (e.g., 35 percent) contain large amounts of water that dilute the chemistry and alter the reactions.
Another problem with the system of FIG. 1 is that it requires a large number of tanks, pumps, mixers, valves, instrumentation, etc., all of which add cost, complexity, and energy consumption to the manufacture, operation, and maintenance of the system.
A further problem with the system of FIG. 1 is that it relies on gravity separation which is inherently slow and inefficient, and still leaves appreciable amounts of contaminants in the purified solution stream flowing back to the flux bath. It also relies on the addition of polymers to settle the suspended solids which, in turn, necessitates the addition of one more tank and mixer to the system.